With conventional wireless communication systems, to improve reception sensitivity, a technology to transmit a signal from a transmitter by using a plurality of antennas and a technology to receive a signal by using a plurality of antennas are employed. These technologies are called “beam forming” or “diversity.”
The beam forming or the diversity technology can reduce deterioration in transmission characteristics caused by noise or interference in order to improve reception sensitivity; however, they are less effective in enhancing a transmission rate.
In contrast, a technology (MIMO) is proposed which can enhance a transmission rate and offers an effect of further improvement in reception sensitivity by using a plurality of antennas.
For example, at IEEE 802.11 which is working on standardization of wireless LAN, a system exceeding 100 Mbps is currently discussed in the task group n, and it is almost certain that MIMO will be adopted. In addition, for the fourth generation mobile communication system, adoption of MIMO is now examined to increase the transmission rate.
However, in building a MIMO transmission system, there is a problem in that IQ mismatch in a quadrature modulator and a quadrature demodulator causes significant impact, thus deteriorating transmission characteristics, as shown in the following non-patent references 1 and 2. The term IQ mismatch means that the respective amplitude conversion rates of an in-phase component and a quadrature component (which correspond to a real part and an imaginary part, respectively, of a complex signal) of a complex signal become different from each other, or the phase difference between the components is away from 90 degrees.
[Non-Patent Reference 1]
Hiroyuki Kamata, Kei Mizutani, Hiraku Sakaguchi, Kiyomichi Araki, “Studies on Characteristic Deterioration of a MIMO Communication System Caused by Incompleteness in the RF System”, Society Convention 2004 B-5-23, The Institute of Electronics, Information and Communication Engineers
[Non-Patent Reference 2]
Hiraku Sakaguchi, Tin Zi Ho, Kiyomichi Araki, “Building of a MIMO Eigen Mode Transmission System and Result of Measurement and Experiments”, The Institute of Electronics, Information and Communication Engineers, Transactions B, vol. J87-B, no. 9, pp. 1454-1466, 2004